WST11 Vascular Targeted Photodynamic Therapy Effect Monitoring by Multispectral Optoacoustic Tomography (MSOT) in Mice.

Objective: Monitoring emerging vascular-targeted photodynamic therapy (VTP) and understanding the time-dynamics of treatment effects remains challenging. We interrogated whether handheld multispectral optoacoustic tomography (MSOT) could noninvasively monitor the effect of VTP using WST11, a vascular-acting photosensitizer, on tumor tissues over time using a renal cell cancer mouse model. We also investigated whether MSOT illumination can induce VTP, to implement a single-modality theranostic approach. Materials and Methods: Eight BalB/c mice were subcutaneously implanted with murine renal adenocarcinoma cells (RENCA) on the flank. Three weeks later VTP was performed (10 min continuous illumination at 753 nm following intravenous infusion using WST11 or saline as control. Handheld MSOT images were collected prior to VTP administration and subsequently thereafter over the course of the first hour, at 24 and 48 h. Data collected were unmixed for blood oxygen saturation in tissue (SO2) based on the spectral signatures of deoxy- and oxygenated hemoglobin. Changes in oxygen saturation over time, relative to baseline, were examined by paired t-test for statistical significance (p < 0.05). In-vivo findings were corroborated by histological analyses of the tumor tissue. Results: MSOT is shown to prominently resolve changes in oxygen saturation in tumors within the first 20 min post WST11-VTP treatment. Within the first hour post-treatment, SO2 decreased by more than 60% over baseline (p < 0.05), whereas it remained unchanged (p > 0.1) in the sham-treated group. Moreover, unlike in the control group, SO2 in treated tumors further decreased over the course of 24 to 48 h post-treatment, concomitant with the propagation of profound central tumor necrosis present in histological analysis. We further show that pulsed MSOT illumination can activate WST11 as efficiently as the continuous wave irradiation employed for treatment. Conclusion: Handheld MSOT non-invasively monitored WST11-VTP effects based on the SO2 signal and detected blood saturation changes within the first 20 min post-treatment. MSOT may potentially serve as a means for both VTP induction and real-time VTP monitoring in a theranostic approach.

Stiffening of rabbit corneas by the bacteriochlorophyll derivative WST11 using near infrared light.

PURPOSE: We evaluated the efficacy and safety of photochemical corneal stiffening by palladium bacteriochlorin 13'-(2-sulfoethyl)amide dipotassium salt (WST11) and near infrared (NIR) illumination, using ex vivo and in vivo rabbit eye models. METHODS: Corneas of post mortem rabbits and living rabbits were pretreated topically with 2.5 mg/mL WST11 in saline or in 20% dextran T-500 (WST-D), washed and illuminated with an NIR diode laser (755 nm, 10 mW/cm(2). Studies with corneas of untreated fellow eyes served as controls. Tensile strength measurements, histopathology, electron spin resonance, and optical spectroscopy and fluorescence microscopy were used to assess treatment effects. Comparative studies were performed with standard riboflavin/ultraviolet-A light (UVA) treatment. RESULTS: WST11/NIR treatment significantly increased corneal stiffness following ex vivo or in vivo treatment, compared to untreated contralateral eyes. The incremental ultimate stress and Young's modulus of treated corneas increased by 45, 113, 115%, and 10, 79, and 174% following 10, 20, and 30 minutes of incubation with WST11, respectively. WST-D/NIR had a similar stiffening effect, but markedly reduced post-treatment edema and shorter time of epithelial healing. WST11/NIR and WST-D/NIR generate hydroxyl and superoxide radicals, but no singlet oxygen in the cornea. Histology demonstrated a reduction in the keratocyte population in the anterior half of the corneal stroma, without damage to the endothelium. CONCLUSIONS: Treatment of rabbit corneas, with either WST11/NIR or WST-D/NIR, increases their biomechanical strength through a mechanism that does not involve singlet oxygen. The WST-D/NIR treatment showed less adverse effects, demonstrating a new potential for clinical use in keratoconus and corneal ectasia after refractive surgery.

Synthesis and evaluation of a stable bacteriochlorophyll-analog and its incorporation into high-density lipoprotein nanoparticles for tumor imaging.

The syntheses of novel near-infrared (NIR) dyes with excellent optical properties in biological tissues have driven the continued improvement of fluorescence imaging of deeply seated tumors. Bacteriochlorophyll a (Bchl), a dye synthesized by the phototrophic bacteria, R. sphaeroids, is particularly suited for deep tissue imaging due to its high absorbance coefficient and good fluorescence quantum yield in the NIR spectrum. However, obstacles that impede the development of this fluorophore are its poor stability and lack of tumor specificity. These issues ultimately limit its utility for tumor detection. Herein we describe a robust synthesis of a novel Bchl analog, bacteriochlorin e(6) bisoleate (BchlBOA), which is chemically stable, has excellent photophysical properties (ex, 752 nm; em, 762 nm) and is tailored for the incorporation into a tumor targetable high-density lipoprotein (HDL)-like nanoparticle (NP). Incorporating BchlBOA into HDL (HDL-BchlBOA) yielded 12 nm sized particles, corresponding well with the diameter of native HDL. Functional cell uptake studies showed that HDL-BchlBOA was taken up by cells expressing the HDL receptor, scavenger receptor B type I (SR-BI), and was inhibited by 25-fold excess native HDL. Furthermore, the NP was successfully detected in KB cancer cells both in vitro and in tumor xenografts. Taken together, these results demonstrate that we successfully synthesized and formulated a stable analog of Bchl that is capable of being incorporated within HDL-like NPs for tumor-targeted imaging.

[Photodynamic therapy: principles and therapeutic indications]. / Principes et applications thérapeutiques de la photothérapie dynamique.

Photodynamic therapy consists in destroying a tumoral or a non tumoral tissue by the effect of both a photosensitizing molecule and a laser light. This simple concept has needed numerous years in order to be used in routine treatments with both photosensitizers and laser light delivered optimally. Researches in chemistry lead to new porphyrin and bacteriochlorophyl derivatives which alleviate the decrease of light absorption by endogenous molecules and in consequence allow a deeper light penetration. Short half-life of these compounds allows an easier treatment monitoring. In parallel, improvements in both laser technology and fibers allow new indications in various pathologies. First applications took place in treatment of respiratory, digestive and urologic cancers. The biggest success to date is recorded in ophthalmology with the treatment of age related macular degeneration. New approaches are explored and clinical studies are ongoing.

Evaluation of the new photosensitizer Stakel (WST-11) for photodynamic choroidal vessel occlusion in rabbit and rat eyes.

PURPOSE: To evaluate the photodynamic potential of a new hydrosoluble photosensitizer (WST-11, Stakel; Steba Biotech, Toussus-Le-Noble, France), for use in occlusion of normal choroidal vessels in the rabbit eye and CNV (choroidal neovascularization) in the rat eye. METHODS: Occlusive and nonocclusive parameters of Stakel and verteporfin photodynamic therapy (PDT) were investigated in pigmented rabbits. Eyes were followed by fluorescein angiography (FA) and histology at various intervals after PDT. RESULTS: When occlusive parameters (fluence of 50 J/cm(2), 5 mg/kg drug dose and DLI [distance to light illumination] of 1 minute) were used, Stakel PDT was efficient immediately after treatment without associated structural damage of the RPE and retina overlying the treated choroid in the rabbit eye. Two days later, total occlusion of the choriocapillaries was seen in 100% of the treated eyes, along with accompanying histologic structural changes in the overlying retina. When the occlusive parameters (fluence, 100 J/cm2; drug dose, 12 mg/m2; and DLI, 5 minutes) of verteporfin PDT were used, occlusion of the choriocapillaries was observed in 89% of the treated eyes. Histology performed immediately after treatment demonstrated structural damage of the overlying retina and RPE layer. Weaker, nonocclusive Stakel PDT parameters (25 J/cm2, 5 mg/kg, and DLI of 10 minutes) did not induce choriocapillary occlusion or retinal lesions on FA or histology. Weaker, nonocclusive verteporfin PDT parameters (10 J/cm2, 0.2 mg/kg, and DLI of 5 minutes) did not induce choriocapillary occlusion. However, histology of these eyes showed the presence of damage in the retinal and choroidal tissues. Moreover, preliminary results indicate that selective CNV occlusion can be achieved with Stakel PDT in the rat eye. CONCLUSIONS: Unlike verteporfin PDT, Stakel PDT does not cause direct damage to the RPE cell layer or retina. These observations indicate that Stakel PDT may have a high potential for beneficial therapeutic outcomes in treatment of AMD.

Characterization of a new photosensitizer (13(2)-hydroxy- bacteriopheophorbide-a methylester) for future treatment of ovarian carcinoma (an experimental study).

UNLABELLED: The photosensitizer 132-hydroxy bacteriopheophorbide-a methyl ester (13(2) OH- BPME) is characterized by a high absorption coefficient at the far red wavelength 750 nm and a good singlet oxygen quantum yield. METHODS & RESULTS: The pharmacokinetics of 132-OH- BPME were studied in ovarian carcinoma on mice after i.v. administration of 7.8 micromole/kg body weight at different incubation intervals. The accumulated dye was chemically extracted from selected tissues and the concentrations were measured by absorption spectroscopy. The parenchymatous organs (liver, spleen and kidney) showed maximum 13(2)- OH- BPME concentrations after 2 hours incubation (liver, spleen), and 4 hours post injection (kidney). A high uptake was detected in the lung with maximum concentration at 2 hours. The malignant tissue accumulated high 13(2)- OH- BPME concentrations between 2-12 hours post injection with peaking at 8 hours. The 13(2)- OH- BPME concentrations in muscle tissue, representing the normal tumour surroundings, and in the skin were very low. CONCLUSION: The results of our study suggest that PDT using 13(2)-OH-BPME could be effective at 8h post injection, where the tumour 13(2)- OH-BPME uptake is maximum and the muscle and skin uptake will be minimum.

Vascular targeted photodynamic therapy with palladium-bacteriopheophorbide photosensitizer for recurrent prostate cancer following definitive radiation therapy: assessment of safety and treatment response.

PURPOSE: Tookad is a novel intravascular photosensitizer. When activated by 763 nm light, it destroys tumors by damaging their blood supply. It then clears rapidly from the circulatory system. To our knowledge we report the first application of Tookad vascular targeted photodynamic therapy in humans. We assessed the safety, pharmacokinetics and preliminary treatment response as a salvage procedure after external beam radiation therapy. MATERIALS AND METHODS: Patients received escalating drug doses of 0.1 to 2 mg/kg at a fixed light dose of 100 J/cm or escalated light doses of 230 and 360 J/cm at the 2 mg/kg dose. Four optical fibers were placed transperineally in the prostate, including 2 for light delivery and 2 for light dosimetry. Treatment response was assessed primarily by hypovascular lesion formation on contrast enhanced magnetic resonance imaging and transrectal ultrasound guided biopsies targeting areas of lesion formation and secondarily by serum prostate specific antigen changes. RESULTS: Tookad vascular targeted photodynamic therapy was technically feasible. The plasma drug concentration was negligible by 2 hours after infusion. In the drug escalation arm 3 of 6 patients responded, as seen on magnetic resonance imaging, including 1 at 1 mg/kg and 2 at 2 mg/kg. The light dose escalation demonstrated an increasing volume of effect with 2 of 3 patients in the first light escalation cohort responding and all 6 responding at the highest light dose with lesions encompassing up to 70% of the peripheral zone. There were no serious adverse events, and continence and potency were maintained. CONCLUSIONS: Tookad vascular targeted photodynamic therapy salvage therapy is safe and well tolerated. Lesion formation is strongly drug and light dose dependent. Early histological and magnetic resonance imaging responses highlight the clinical potential of Tookad vascular targeted photodynamic therapy to manage post-external beam radiation therapy recurrence.

Selection of dosing regimen with WST11 by Monte Carlo simulations, using PK data collected after single IV administration in healthy subjects and population PK modeling.

WST11, a novel generation of photo sensitizers to be used for vascular-targeted phototherapy (VTP), is effective at short interval between injection and illumination and it is expected to enable selective destruction of neovasculature with minimal side effects or skin photo toxicity. This study was conducted to evaluate the clinical and laboratory safety, tolerability and pharmacokinetic profile of WST11 given as a single intravenous administration (1.25, 2.5, 5, 7.5, 10, or 15 mg/kg) during an escalating dose study in healthy male subjects. This article describes WST11 population pharmacokinetic modeling and simulations performed to optimize the IV infusion-dosing regimen in combination with illumination, the target PK profile being plateau concentrations during approximately 30 min. The study included 42 healthy male subjects, administered 1.25, 2.5, 5, 7.5, 10, or 15 mg/kg as a 10-min IV infusion. A population pharmacokinetic model was developed using nonlinear mixed effects modeling (NONMEM). Monte Carlo simulations of the population PK dataset (NONMEM) were performed to select series of dosing regimen which would result in a plateau of concentration lasting at least 30 min and allow laser illumination. A two-compartment model with nonlinear elimination best described the data. No demographic factor was shown to affect the WST11 pharmacokinetics. The clearance was shown to decreases with the dose administered, ranging from 6 L/h (dose of 79 mg) to 2 L/h (dose of 1110 mg). The duration of the infusion was estimated at 12 min. The volume of distribution of the central compartment was 3 L and the volume of the peripheral compartment was 1.15 L. The apparent inter-compartmental clearance was 0.137 L/h. The between subjects variability on clearance and on volume was low. Residual variability was moderate with a CV of 21%. Due to the dose effect on clearance and the rapid elimination, simulations showed that different dosing inputs are necessary: for 5 and 10 mg/kg BW, a sufficiently good dosing scenario is to administer 80% of the dose over 5 min, 15% over 10 min and the remaining 5% over 10 min. For lower doses, the sequence 70% in 5 min/20% in 10 min/10% in 10 min is preferable. The pharmacokinetic profile of WST11 by IV administration would allow a treatment by laser illumination in good clinical conditions using controlled infusions. The study results do not indicate that the dose should be adjusted for body size. The only factor that determines the drug input profile is the dose level, since the elimination half-life decreases when the dose administered increases. The use of the population PK model for simulations has shown that, at dose level of 5 mg/kg or more, a loading dose of 80% dose given over 5 min followed by 15% of dose during 10 min and remained dose to give over 10 min would result in a favorable PK profile.

Study of manganese bacteriopheophorbide as a potential contrast agent for magnetic resonance tomography.

The use of manganese bacteriopheophorbide characterized by a high relaxation capacity and selectively accumulating in the tumor as a contrast agent for magnetic resonance tomography significantly improves tumor contrasting against the background of normal tissues. The pharmacokinetics and selectivity of accumulation were studied by diffuse reflection spectroscopy.

Photodynamic therapy with WST09 (Tookad): quantitative studies in normal colon and transplanted tumours.

Photodynamic therapy (PDT) is attracting increasing interest for the safe destruction of localised tumours in a range of organs. However, most photosensitising drugs require a delay of hours to days between drug administration and light activation with skin photosensitivity that may last for weeks. WST09 (Tookad) is a new faster acting photosensitiser that clears within a few hours. In normal rat colon, after sensitisation with an intravenous bolus of WST09, light was delivered to a single point on the mucosa and the extent of PDT necrosis measured 3 days later. The lesion diameter was greatest with the highest dose of drug and light and the shortest drug light interval (DLI), falling rapidly with a DLI more than 5 min. In tumours transplanted subcutaneously or into the colon, the extent of necrosis only started falling with a DLI greater than 15 min, suggesting a possible window for tumour selectivity. Histological changes 3 days after PDT were essentially the same as those seen with longer acting photosensitisers. The lesion dimensions were comparable to the largest ones seen with other photosensitisers under similar experimental conditions. We conclude that WST09 is a powerful photosensitiser that produces PDT effects similar to those seen with longer acting drugs, but with the major advantages of a short DLI and rapid clearance.

Techniques for delivery and monitoring of TOOKAD (WST09)-mediated photodynamic therapy of the prostate: clinical experience and practicalities.

Photodynamic therapy of solid organs requires sufficient PDT dose throughout the target tissue while minimizing the dose to proximal normal structures. This requires treatment planning for position and power of the multiple delivery channels, complemented by on-line monitoring during treatment of light delivery, drug concentration and oxygen levels. We describe our experience in implementing this approach in Phase I/II clinical trials of the Pd-bacteriophephorbide photosensitizer TOOKAD (WST09)-mediated PDT of recurrent prostate cancer following radiation failure. We present several techniques for delivery and monitoring of photodynamic therapy, including beam splitters for light delivery to multiple delivery fibers, multi-channel light dosimetry devices for monitoring the fluence rate in the prostate and surrounding organs, methods of measuring the tissue optical properties in situ, and optical spectroscopy for monitoring drug pharmacokinetics of TOOKAD in whole blood samples and in situ in the prostate. Since TOOKAD is a vascular-targeted agent, the design and implementation of the techniques are different than for cellular-targeted agents. Further development of these delivery and monitoring techniques will permit full on-line monitoring of the treatment that will enable real-time, patient-specific and optimized delivery of PDT.

Studies of a vascular-acting photosensitizer, Pd-bacteriopheophorbide (Tookad), in normal canine prostate and spontaneous canine prostate cancer.

BACKGROUND AND OBJECTIVES: Photodynamic therapy (PDT) mediated with Tookad (Pd-bacteriopheophorbide, WST09) was investigated pre-clinically as part of a program to develop an alternative modality for treating prostate cancer. STUDY DESIGN/MATERIALS AND METHODS: Spontaneous canine prostate cancer and normal canine prostate were used as the animal models. Interstitial PDT was performed by IV infusion of the photosensitizer and irradiating the prostates with a diode laser (763 nm). The prostates were harvested 1-week post-PDT and subjected to histopathologic examinations. The effects of the drug doses and light doses were studied for one- and two-session PDT. Pharmacokinetics were studied using HPLC assay. The feasibility of using perfusing CT scans for assessing PDT lesions was also evaluated. RESULTS: Tookad is a vascular-acting drug and clears rapidly from the circulation. Tookad-PDT-induced lesions, in both normal and cancerous prostates, were characterized by marked hemorrhagic necrosis. CONCLUSIONS: Tookad-PDT is very effective in ablating prostatic tissue through its vascular effects.

WST11, a novel water-soluble bacteriochlorophyll derivative; cellular uptake, pharmacokinetics, biodistribution and vascular-targeted photodynamic activity using melanoma tumors as a model.

WST11 is a novel negatively charged water-soluble palladium-bacteriochlorophyll derivative that was developed for vascular-targeted photodynamic therapy (VTP) in our laboratory. The in vitro results suggest that WST11 cellular uptake, clearance and phototoxicity are mediated by serum albumin trafficking. In vivo, WST11 was found to clear rapidly from the circulation (t1/2=1.65 min) after intravenous bolus injection in the mouse, whereas a longer clearance time (t1/2=7.5 min) was noted in rats after 20 min of infusion. The biodistribution of WST11 in mouse tissues indicates hepatic clearance (t1/2=20 min), with minor (kidney, lung and spleen) or no intermediary accumulation in other tissues. As soon as 1 h after injection, WST11 had nearly cleared from the body of the mouse, except for a temporal accumulation in the lungs from which it cleared within 40 min. On the basis of these results, we set the VTP protocol for a short illumination period (5 min), delivered immediately after WST11 injection. On subjecting M2R melanoma xenografts to WST11-VTP, we achieved 100% tumor flattening at all doses and a 70% cure with 9 mg/kg and a light exposure dose of 100 mW/cm2. These results provide direct evidence that WST11 is an effective agent for VTP and provide guidelines for further development of new candidates.

Determination of the in vivo pharmacokinetics of palladium-bacteriopheophorbide (WST09) in EMT6 tumour-bearing Balb/c mice using graphite furnace atomic absorption spectroscopy.

Palladium-bacteriopheophorbide (WST09), a novel bacteriochlorophyll derivative, is currently being investigated for use as a photodynamic therapy (PDT) drug due to its strong absorption in the near-infrared region and its ability to efficiently generate singlet oxygen when irradiated. In this study, we determined the pharmacokinetics and tissue distribution of WST09 in female EMT6 tumour-bearing Balb/c mice in order to determine if selective accumulation of this drug occurs in tumour tissue. A total of 41 mice were administered WST09 by bolus injection into the tail vein at a dose level of 5.0 +/- 0.8 mg kg(-1). Three to six mice were sacrificed at each of 0.08, 0.25, 0.5, 1.0, 3.0, 6.0, 9.0, 12, 24, 48, 72, and 96 h post injection, and an additional three control mice were sacrificed without having been administered WST09. Terminal blood samples as well as liver, skin, muscle, kidney and tumour samples were obtained from each mouse and analyzed for palladium content (from WST09) using graphite furnace atomic absorption spectroscopy (GFAAS). The representative concentration of WST09 in the plasma and tissues was then calculated. Biphasic kinetics were observed in the plasma, kidney, and liver with clearance from each of these tissues being relatively rapid. Skin, muscle and tumour did not show any significant accumulation at all time points investigated. No selective drug accumulation was seen in the tumour and normal tissues, relative to plasma. Thus the results of this study indicate that vascular targeting resulting from WST09 in the circulation, as opposed to selective WST09 accumulation in tumour tissues, may be responsible for PDT effects in tumours that have been observed in other WST09 studies.

Indirect detection of photosensitizer ex vivo.

Photodynamic therapy induces the production of reactive oxygen species (ROS) within tissues exposed to laser light after administration of a sensitizer. In the context of continuing clinical and commercial development of chemicals with sensitizing properties, a minimally invasive assay is needed to determine the tissue kinetics of fluorescent or non-fluorescent photoreactive drugs. The level of ROS was determined ex vivo from 1 mm3 biopsy samples using 2'-7' dichlorofluorescin diacetate (DCFH-DA), a fluorescent probe which was converted into highly fluorescent dichlorofluorescein (DCF) in the presence of ROS. This assay was tested on meta(tetrahydroxyphenyl)chlorin (m-THPC, FOSCAN), a powerful and fluorescent sensitizer, and bacteriochlorophyll derivative WST09 (TOOKAD), a near-infrared absorbing sensitizer that is only slightly fluorescent. In conjunction with the ROS assay, the tissue accumulation of m-THPC was determined on biopsy samples using an optic fibre spectrofluorometer (OFS). DCF fluorescence was proportional to the level of oxidation induced by horseradish peroxidase used as a control and to the concentration (range: 0-5 microg x ml(-1)) of both selected photosensitizers irradiated in a tube together with DCFH. Regardless of the organ studied, an excellent correlation was found between fluorescence measurement by OFS and ROS determination for m-THPC. m-THPC (2 mg x kg(-1) iv) accumulation in tumour tissues was best after 48 h, and the best signal was obtained in liver. With non-fluorescent WST09 (2 mg x kg(-1)), ROS determination showed the best tumour uptake 48 h after injection, with a tumour/muscle ratio of 5.4. The ROS assay appears to be feasible for determining sensitizer concentration in regular grip biopsy tissue samples.

13(2)-hydroxy-bacteriopheophorbide a methyl ester pharmacokinetics measurements with fluorescence versus absorption spectroscopy. Is there a difference?

OBJECTIVE: Quantification of photosensitizer concentration in tissue improves the planning and, subsequently, the outcome of photodynamic therapy. This study was designed to determine if the method of photosensitizer evaluation influences the accuracy of pharmacokinetic results. MATERIALS AND METHODS: In vivo 13(2)-Hydroxy-bacteriopheophorbide a methyl ester (13(2)-OH-BPME) pharmacokinetics in mice bearing LEWIS lung carcinoma was studied using fluorescence in situ and absorption spectroscopy (following photosensitizer chemical extraction) as photosensitizer quantification methods. The correlation of 13(2)-OH-BPME fluorescence intensity and its concentration using the absorption spectroscopy were determined for each tissue. RESULTS: The parenchymatous organs showed maximum 13(2)-OH-BPME concentration and fluorescence intensity at 2 h post-injection (lung, liver, spleen), and at 2 h post-injection in the kidney with both quantification methods (identical correlation). There was a difference in the time of maximum photosensitizer fluorescence intensity and its concentration in tumor, muscle, and skin (low correlation). The time of maximum fluorescence intensity in muscle, skin, and tumor was at 4 h, 12 h, and 12 h post injection respectively while its maximum concentration was at 2 h, 4 h, and 8 h post-injection. CONCLUSION: The method of photosensitizer evaluation affects the accuracy of its pharmacokinetic results.

Serine conjugates of chlorophyll and bacteriochlorophyll: photocytotoxicity in vitro and tissue distribution in mice bearing melanoma tumors.

Chlorophyll (Chl) and bacteriochlorophyll (Bchl) have been made water soluble by transesterification with serine (Ser) at the propionyl residue and tested as potential reagents for photodynamic therapy (PDT). Photocytotoxicity of the conjugates Chl-Ser and Bchl-Ser in M2R mouse melanoma was tested in cell cultures. Tissue uptake and clearance of the photosensitizers in CD1 nude and C57B1 mice implanted with M2R tumors are described. Photocytotoxicity in cell cultures was determined microscopically and by [3H]thymidine incorporation. The LD50 values in vitro were 0.05-0.1 microM for both sensitizers while that of the commercially available hematoporphyrin derivative (HPD, Photosan) was over 100 times higher for the same light intensity (45 mW/cm2). Pigment concentrations were determined fluorometrically in acetone extracts of the tissues of interest at different times after intraperitoneal injection of 20 mg pigment/kg body weight. The distribution pattern of Chl-Ser in the different tissues resembled that reported for Photofrin, chlorin and bacteriochlorin derivatives. Clearance from normal tissues was essentially completed within 16 h for Bchl-Ser and 72 h for Chl-Ser with mean half-lives (t 1/2) of about 2 and 7 h, respectively. In contrast, the clearance rates of these pigments and their metabolites from melanoma tumor tissue were significantly longer: t 1/2 = 20 h for Chl-Ser and 15 h for Bchl-Ser and metabolites. The clearance rates showed biphasic or single exponential decay patterns in normal tissues and in tumors, respectively. Cumulatively the high phototoxicity, simple mode of delivery and fast tissue clearance rates reported here suggest that polar conjugates of Chl and Bchl promise to be highly effective PDT reagents.